The present invention relates to a knitting machine comprising at least one needle bed having slidably mounted in its grooves corresponding needles formed with at least one catch or heel adapted to be positioned at two levels with respect to the top surface of the needle bed, that is, one level in which each needle catch can be driven by the cams or cam holders and another level in which the needle catch is clear of the path of said cams; means for acting upon said catches and moving them to the positions corresponding to said two levels, one of said means consisting of a cam adapted to depress said catches toward the lower level, and key means associated with each needle for moving the needles to their operative position in which they can be driven by the working cams.